Power control during a transmission pause

ABSTRACT

A transmitting station performs closed loop power control prior to a transmission pause. A closed loop transmission power level prior to the pause is determined. A reference signal is received and a received power level of the reference signal prior to and during the transmission pause is determined. The measured reference signal received power levels are compared to a transmit power level of the reference signals to produce a pathloss estimate of the reference signal prior to and during the transmission pause. A new transmit power level is determined by adjusting the closed loop transmission power level by a change between the prior to and during pathloss estimates. A transmission power level of the transmitting station is set to the determined new transmit power level. A communication is transmitted at the set transmission power level.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/325,772 filed Dec. 19, 2002, now U.S. Pat. No. 6,748,235 which claimsthe benefit of U.S. provisional application No. 60/425,607 filed Nov.12, 2002, which are incorporated by reference as if fully set forth.

BACKGROUND

This invention generally relates to spread spectrum communicationsystems. More particularly, the present invention relates to controllingtransmission power levels in such systems.

In many spread spectrum communication systems, multiple usercommunications are sent over a shared frequency spectrum. In codedivision multiple access (CDMA) communication systems, the multiplecommunications are sent using different codes to distinguish them. Intime division duplex (TDD)/CDMA and time division synchronous CDMA(TD-SCDMA) communication systems, the spectrum is also time divided tobetter utilize the spectrum.

Since multiple communications are sent in the same spectrum, thecommunications may interfere with each other. To reduce the interferencethat the communications induce on one another, adaptive power control isused. The purpose of adaptive power control is to transmit eachcommunication at a minimum power level to achieve a specified receptionquality, such as a target signal to interference ratio (SIR).

One scheme to implement adaptive power control is closed loop powercontrol. In closed loop power control, a transmitter transmits acommunication at a certain power level. A target receiver receives thecommunication and measures the SIR of the received signal. The measuredSIR is compared to a target SIR. If the measured SIR is larger than thetarget SIR, a power command indicating that the transmitter shoulddecrease its transmission power is sent. If the measured SIR is lessthan the target SIR, a power command indicating that the transmittershould increase its power level is sent. The power command is,typically, a single bit indicating either an increase or decrease inpower level. The transmitter receives the power command and changes itstransmission power level in response to the power command by a fixedamount, such as by increasing or decreasing the transmission power levelby 1 dB.

When there is a pause in the transmissions, no data is present in thechannel for SIR calculations. In this situation, the receiver maygenerate either no power commands or dummy power commands. The dummycommands are arbitrary commands and most likely have no relation to anyactual change in channel conditions. Likewise, when there is a pause inthe receiver's transmissions, no power commands are sent to thetransmitter. Since the channel conditions may change during thesepauses, the transmission power level may deviate from the desired powerlevel for the current channel conditions. If the power level is too low,a desired quality of service (QOS) may not be achieved. If the powerlevel is too high, unnecessary interference may be introduced onto otherusers.

Accordingly, it is desirable to have other approaches to power control.

SUMMARY

A transmitting station of the present invention performs closed looppower control prior to a transmission pause and determines the closedloop transmission power level. A reference signal is received and areceived power level of the reference signal prior to and during thetransmission pause is determined. The measured reference signal receivedpower levels are compared to a transmit power level of the referencesignals to produce a pathloss estimate of the reference signal prior toand during the transmission pause. A new transmit power level isdetermined by adjusting the closed loop transmission power level by achange between the prior to and during pathloss estimates. Atransmission power level of the transmitting station is set to thedetermined new transmit power level. A communication is transmitted atthe set transmission power level.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a flow chart of an embodiment of power control during atransmission pause.

FIG. 2 is a simplified block diagram of a transmitting and receivingstation using power control during a transmission pause.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention will be described with respect to the figureswherein like numerals represent like elements throughout. Although powercontrol during a transmission pause is explained with the preferredTDD/CDMA and TD-SCDMA communication systems, it can be applied to othercommunication systems experiencing channel reciprocity between theuplink and downlink.

A preferred embodiment of power control during a transmission pause isexplained using the flow chart of FIG. 1 and the components of twosimplified communication stations of FIG. 2. For the followingdiscussion, the communication station having its transmission powerlevel controlled is referred to as the transmitting station 20 and thecommunication station receiving the power controlled communications isreferred to as the receiving station 22. Since power control during atransmission pause can be used for uplink, downlink or both types ofcommunications, the transmitting station 20 may be a base station, auser equipment or both.

When data is transmitted by the transmitting station 20, data to betransmitted to the receiving station 22 is modulated and spread toproduce a spread spectrum radio frequency (RF) signal by a modulationand spreading device 24. An amplifier 26 is used to control the powerlevel of the RF signal. The RF signal is radiated by an antenna 36 orantenna array through a wireless radio interface 38 to the receivingstation 22.

The RF signal is received by an antenna 40 or antenna array of thereceiving station 22. The impulse response of the received signal isestimated by a channel estimation device 44. In a TDD/CDMA or TD-SCDMAcommunication system, the channel estimation is performed using knowntraining sequences in a midamble portion of a communication burst. Usingthe impulse responses, the data detection device 42 recovers thetransmitted data from the received RF signal. Using information fromboth the data detection device 42 and the channel estimation device 44,a quality of the received RF signal, such as a SIR, is measured by aquality measurement device 50. A transmit power command (TPC) generationdevice compares the measured quality to a target quality. If themeasured quality is greater than the target, a decrease command isgenerated. If the measured quality is less than or equal to the target,an increase command is generated. Although other commands may be used,the preferred command is a single bit with a +1 indicating an increasein power level and a −1 indicating a decrease in power level. The TPC issent to the transmitting station 20. If the transmitting station 20 isnot transmitting, the receiving station 22 may send no TPCs to thetransmitting station 20 or dummy (arbitrary) TPCs. If the receivingstation 22 has paused its transmissions, no TPCs are transmitted to thetransmitting station 20 during the pause.

To compensate for the lack of TPCs, whether due to a pause in thetransmitting or receiving station's transmissions, a pathloss estimateis used where the pathloss estimate includes all radio channelconditions between the transmitting and receiving stations, such as fastfading, slow fading and long-term fading. At the receiving station 22,reference data is modulated and spread by a modulation and spreadingdevice 48 to produce a reference RF signal. The reference RF signal isradiated by the antenna 40 or antenna array to the transmitting station20 through the wireless radio interface 38, (step 64). For uplink powercontrol for a third generation partnership project 3GPP TDD/CDMA system,the preferred reference channel is the primary common control physicalchannel (P-CCPCH), the secondary common control physical channel(S-CCPCH), or the forward access channel (FACH). This invention can useas a reference channel any physical channel with known transmit power.

The transmitting station 20 receives the reference signal using itsantenna 36 or antenna array, (step 60). A reference signal powermeasurement device 34 measures the power level of the received referencesignal, (step 62). Using the measured reference signal power, a pathlossdetermination device 32 determines the pathloss for the referencesignal, (step 64). The transmission power level of the reference signalmay be known a priori by the transmitting station 20, or maybe signaledto the transmitting station 20, such as by using a broadcast channel(BCH). The pathloss is determined by subtracting the received powerlevel from the transmitted power level.

When there are no pauses in transmission from either the transmittingstation 20 or receiving station 22, the transmit power control device 28determines the transmit power level using a closed loop algorithm. TheTPC is received by a TPC receive and judgment device 30 and the value ofthe TPC is determined. Using the determined value for the TPC, thetransmission power level is either increased or decreased by thetransmit power control device 28 by a fixed amount, such as 1 dB. Thetransmit power control device 28 sets the power level of the transmittedRF signal by adjusting a gain of the amplifier 26. When there is a pausein transmission of the dedicated channel from either the transmittingstation 20 or the receiving station 22, the transmission power level isset using the closed loop transmission power level prior to the pause, apathloss estimate prior to the pause and a recent path loss estimatederived from a reference signal, (step 66). The common channel (P-CCPCH)is always sent.

When the receiving station 22 pauses its transmissions which carry thetransmit power commands (TPCs), the transmit power control device 28ignores any dummy TPCs which may be generated by the receiver. Onemethod for determining the transmission power level of the transmittingstation 20 for either a TDD/CDMA or TD-SCDMA communication system is setforth in Equation 1.P(k)=P _(beforeDTX) +L _(beforeDTX) −L(k);  Equation 1where P(k) is the transmission power level in dBm in the k^(th) frame orsub-frame in the pause; P_(beforeDTX) is the transmission power in dBmin the last frame before the pause (the discontinued transmission);L_(beforeDTX) is the pathloss in dB in the last frame or sub-framebefore the pause; and L(k) is the pathloss in dB in the k^(th) frame orsub-frame during the pause.

The change in pathloss, L_(beforeDTX)−L(k), is an indication of thechange in the channel conditions. By adjusting the power level justprior to the pause by the change in pathloss, the new transmission powerlevel compensates for the changes in the channel conditions.

Alternately, Equation 1 can be expressed such as per Equation 2:P(k)=P(k−1)+L(k−1)−L(k);  Equation 2where P(k−1) is the transmission power level in dBm in the (k−1)th frameor sub-frame in the pause; L(k−1) is the pathloss in dB in the (k−1)thframe or sub-frame during the pause.

Preferably, to better reflect the quality of the pathloss estimates, thepathloss estimates are weighted against a long term pathloss estimate,L_(O) to produce a weighted pathloss. As time passes, the accuracy ofthe most current estimate diminishes and the long term pathloss L₀becomes a better estimate of the true pathloss. As the most currentpathloss estimate becomes stale, the long term pathloss is given moreweight. An example of weighted pathloss and can be seen in Equation 3:L _(w)(k)=αL(k)+(1−α)L _(O);  Equation 3where L_(w) (k) is the weighted pathloss in dB in the k^(th) frame orsub-frame during the pause; L(k) is the pathloss in dB in the k^(th)frame or sub-frame during the pause; L_(O) is the long term pathloss;and α is a weighting factor which is 0<=α<=1. For time slot basedcommunication systems, such as TDD/CDMA and TD-SCDMA, α is based on thenumber of time slots, ΔTS, between the time that the pathloss in thelast frame or sub-frame before the pause was estimated and the time thatthe pathloss in the k^(th) frame or sub-frame during the pause isestimated, such as per Equation 4:

$\begin{matrix}{{\alpha = {\exp\left( {- \frac{\Delta\; T\; S}{C}} \right)}};} & {{Equation}\mspace{14mu} 4}\end{matrix}$where C is the constant representing the degree of a weighting.

The preferred equation for determining the transmission power level foreither a TDD/CDMA or TD-SCDMA communication system, when there is apause in transmission from the transmitting station 20 is illustrated byEquation 5:.P _(afterDTX) =P _(beforeDTX) +L _(beforeDTX) −L _(lastDTX);  Equation 5where P_(afterDTX) is the initial transmission power level in dBm afterthe pause. P_(beforeDTX) is the transmission power in dBm in the lastframe before the pause. L_(beforeDTX) is the pathloss in dB in the lastframe or sub-frame before the pause. L_(lastDTX) is the pathloss in dBin the last frame during the pause. In this case, the transmit powercontrol device 28 ignores any dummy TPCs it receives.

To better reflect the quality of the pathloss estimates, preferably, thepathloss estimates are weighted against a long term pathloss estimate L₀such as per Equation 2.

1. A method for adjusting a transmission power level of a transmitterafter commencement of a discontinuous transmission, the methodcomprising: receiving a reference signal during discontinuoustransmission; determining a change in channel conditions duringdiscontinuous transmission using the reference signal; adjusting thetransmission power level in response to the change in channelconditions.
 2. The method of claim 1 wherein the change in channelconditions is determined by comparing an actual path loss before thediscontinuous transmission and an estimated path loss during thediscontinuous transmission calculated using the reference signal.
 3. Themethod of claim 2 wherein the estimated path loss is weighted using along term path loss estimate.
 4. A transceiver configured to adjust atransmission power level after commencement of a discontinuoustransmission, the transmitter comprising: a receiver for receiving areference signal during the discontinuous transmission; a measurementunit for determining a change in channel conditions during thediscontinuous transmission using the reference signal; a power controlunit for adjusting the transmission power level in response to thechange in channel conditions.
 5. The transceiver of claim 4 wherein themeasurement unit determines the change in channel conditions bycomparing an actual path loss before the discontinuous transmission andan estimated path loss during the discontinuous transmission calculatedusing the reference signal.
 6. The transceiver of claim 5 wherein themeasurement unit determines a weighted estimated path loss using a longterm path loss estimate.